1. Field of the Invention
This invention relates to a signal processing circuit of an electrostatic capacity type touch panel.
2. Description of the Related Art
The electrostatic capacity type touch sensor has been known as a data input device to various kinds of electronic devices such as a mobile phone, a portable audio device, a portable game console, a television and a personal computer.
A conventional signal processing circuit of an electrostatic capacity type touch panel is explained referring to FIG. 10 and FIG. 11. A sense line 61 (touch pad) that has an electrostatic capacitor 62 having a capacitance C is disposed on a touch panel 60, as shown in FIG. 10.
The sense line 61 is connected to a non-inverting input terminal (+) of a differential amplifier 63 (comparator) through a wiring 64. A reference voltage Vref is applied to an inverting input terminal (−) of the differential amplifier 63. A constant current power supply 65 is connected to the wiring 64 that connects between the sense line 61 and the non-inverting input terminal (+) of the differential amplifier 63.
Operations of the signal processing circuit of the electrostatic capacity type touch panel are explained referring to FIG. 11. When a finger 66 of an operator is far away from the sense line 61, a capacitance associated with the sense line 61 is C. In this case, a voltage at the sense line 61 increases from 0 V in a reset state as the electrostatic capacitor 62 connected with the sense line 61 is charged by a constant current from the constant current power supply 65. An output voltage of the differential amplifier 63 is inverted when the voltage at the sense line 61 reaches the reference voltage Vref. A length of time from the reset state to the inversion of the differential amplifier 63 in this case is referred to as t1.
When the finger 66 of the operator approaches the sense line 61, on the other hand, the capacitance associated with the sense line 61 increases to C+C′. The increment C′ is a capacitance of a capacitor formed between the finger 66 of the operator and the sense line 61. As a result, the length of time that the voltage at the sense line 61 takes from 0 V to the reference voltage Vref increases to t2 (t2>t1). That is, it is possible to detect whether the finger 66 of the operator has touched the sense line 61 or not, based on a difference (t2−t1) in the length of time taken by the transition from the reset state to the inversion of the differential amplifier 63.
Technologies mentioned above are disclosed in Japanese Patent Application Publication No. 2005-190950, for example.
However, the signal processing circuit described above is a single input type in which a signal from the single sense line 61 is inputted to the differential amplifier 63, and has a problem that the voltage at the sense line 61 is varied to cause malfunctioning when a noise is applied to the sense line 61.
On the other hand, a differential input type signal processing circuit in which a difference between capacitances associated with two sense lines is detected by an electric charge amplifier is tolerant of the noise and is capable of forming a high sensitivity touch sensor. The differential input type signal processing circuit is suitable for a single-touch, which means that only one sense line is touched, but has a problem that a touch position can be not detected in the case of a multi-touch, which means that two or more than two sense lines are simultaneously touched. The problem is caused because the difference between the capacitances of the two sense lines is lost.